Detectors are known, that have a layer of metallic phthalocyanine for contact with the gases issuing from combustion, for example of an internal combustion engine, in order to measure an overall nature or proportions of the combustion gases. The electrical resistance of the layer of phthalocyanine, which is a semi-conductor, varies with the nature and proportions of the combustion gases, which depends, in turn, on the respective proportions of air and fuel combined in the combustion.
Such a detector emits an electrical signal which depends overall on the mixture of gases in the combustion gases and which may be used to indicate the ratio between the quantity of air and of fuel and/or to optimalize this ratio when the signal delivered by the detector is used in an open-loop which regulates the quantity of air or the quantity of fuel.
U.S. Pat. No. 4,381,922 describes a combustion detectors having an insulating plate, electrodes deposited on this plate and a thin layer of an amorphous or crystalline metallo-phthalocyanine deposited between said electrodes. The thin layer is composed of particles of iron(II), iron(III), nickel, cobalt or copper phthalocyanine, which were suspended in a liquid solvent selected from the group of carbon tetrachloride, ether and acetone in order to be applied in the thin layer.
This Patent teaches in particular that suspending a copper or iron phthalocyanine for 24 hours in carbon tetrachloride or in a chlorinated solvent produces a compound having an electrical resistance that is ten times lower than that of the same metallic phthalocyanine not treated with the carbon tetrachloride or chlorinated solvent.
This Patent also teaches the use of a detector having such a sensitive layer of copper phthalocyanine treated with ether to monitor correct functioning of a burner. The curve of variation of the resistance as a function of excess combustion air passes through a fairly flat minimum near the stoichiometric proportions, however. Nevertheless the detector may be used for regulating the admission of air in to the burner.
It also is known that the conductivity of the phthalocyanines is also a function of the hygrometric degree of the atmosphere in contact with which they are placed. This property is used for constructing hygrometers based on phthalocyanines.
The electrical properties of the phthalocyanines are explained as follows. At ambient temperature and in the absence of gas absorbed on the surface, the phthalocyanines are semi-conductors of type p.
In the presence of gaseous molecules having an electron receiver effect, which is the case of the majority of oxygenated gases issuing from a combustion such as oxygen or oxides of nitrogen, sulfur or carbon, the formation of positive charge carriers is promoted. In fact, the transfer of an electron towards a molecule of phthalocyanine on which an oxygenated gas is adsorbed is facilitated by the electron receiver effect of the gas.
This property explains that the phthalocyanines may be used to detect the overall nature of the combustion gases.
On the other hand, water vapour is an electron donor gas which reduces the conductivity of the phthalocyanine. The positive charges are stabilized by the water vapour and the number of positive carriers capable of ensuring conduction decreases.
For sufficient water vapour contents, the semi-conductivity may become of n type.
The action of water vapour or of any other electron donor gas is manifested especially in the presence of a previously absorbed electron receiver gas. In that case, the adsorbed water vapour progressively annihilates the electron receiver effect of oxygen.
The gases produced by combustion, particularly by the combustion of hydrocarbons, forcibly contain water vapour which results from the combination of the hydrogen of the hydrocarbons with the oxygen of the air.
The foregoing brief statement shows that, in order to use phthalocyanines for qualitatively or quantitatively detecting the nature of the oxygenated gases resulting from a combustion, it is necessary to eliminate or considerably reduce the effect of the water vapour contained in the gases, otherwise the presence of water vapour will lead to variations in electrical resistance which do not correspond to the presence of oxygenated gases and which falsify the detection.
U.S. Pat. No. 4,381,922 teaches that, when a detector based on phthalocyanines is used for monitoring combustion in a burner in order to regulate the ratio between the quantities of air and of fuels, the detector is placed in a cell heated to a temperature of 95.degree. C., in order to avoid condensation on the detector of water, coming from the combustion by maintaining the detector at a temperature higher than the dew point.
It is imperative that the detectors based on phthalocyanines be maintained at a temperature higher than the dew point. In fact, if conducting water condenses on the detector, the electrodes thereof are short-circuited and the electrical signal between electrodes is not a function of the nature of the combustion gases.
However, it is not sufficient to eliminate the condensation of water on the detector. The effect of the water vapour contained in the combustion gases which intervenes even in the absence of condensation must also be eliminated or considerably reduced.
U.S. Pat. No. 4,381,922 teaches that one of the problems encountered when detectors based on phthalocyanines are used, is the sensitivity thereof to the humidity of the atmosphere. It teaches a means for solving this problem which is to add silica gel or a molecular sieve finely ground and saturated with water in a mixture of phthalocyanines and of carbon tetrachloride used for manufacturing the detectors. The silica gel or molecular sieve powder then acts as a buffer which regularizes and stabilizes the reaction of the detectors to humidity.
FIG. 10 of said U.S. Patent shows that the logarithm of the resistance of a detector obtained by this process passes through a minimum for a value of the excess of combustion air close to stoichiometry, but this minimum is relatively flat, as indicated above. Such flattening of the curve of variation of the resistance near the minimum is due in particular to the addition of silica gel or a molecular sieve which fix the water. This causes the variations in resistance due to a change in composition of the gases to be masked by the action of the water on the phthalocyanines.
This slow variation of the resistance on either side of the minimum is not propitious to optimalization of combustion.
In fact, the water content fixed in the preparation which flattens the minimum of the curve representing the logarithm of the resistance as a function of the excess of air prevents the operational optimum from being distinguished. It has now been ascertained that the values of resistance are substantially identical at +3 and -3% of excess of air. This characteristic is highly detrimental to an optimalization of combustion as, at -3% (negative excess of air), the burner produces much non-burned carbon monoxide and hydrocarbon, which causes considerable pollution and runs risks of explosion.